1. Field of the Invention
The invention relates to low-frequency electromagnetic absorption surfaces.
2. Discussion of Prior Art
Surface plasmon polaritons (SPPs) are charge density oscillations induced at the surface of a metal at a metal-dielectric interface when photons are coupled to the mode in the correct manner. The momentum of the incident photons must be boosted if the resonant condition is to be met, and this can be achieved by corrugating the metal to form a diffraction grating. The energy is absorbed by the metal due to damping of the charge density oscillation (i.e. charge collisions lead to heating in the metal), and hence the plasmons cannot convert back to photons for re-emission. In this manner the reflectivity of the metal is reduced when photons are absorbed. This phenomenon is well known at visible frequencies, and forms the basis of many sensor designs.
At microwave frequencies any SPPs that are excited at the surface of the metal will propagate without loss because the charge density oscillations are virtually undamped (i.e. the photon energy cannot be absorbed). Instead of being absorbed, the SPPs will skim the surface until they are converted back to photons at a diffractive feature such as an edge, a curve or the original diffraction grating. Hence the radiation will eventually be re-emitted, and possibly back towards the radiation source. In order to reduce these stray emissions, lossy materials are used as surface coatings to absorb any SPPs that are excited, and methods to prevent the excitation of the modes are sought.
A flat metal plate is a highly efficient microwave reflector that will not normally support SPPS. If it is desired that the plate should absorb all of the energy that fails upon it then absorbing materials are used as surface coatings. Electrically-absorbing materials need to be placed at specific distances from the metal. the shortest of which is a quarter of the wavelength to be absorbed. In the case of magnetic absorbers these are placed directly onto the metal plate. but they are far heavier than electric absorbers. Hence weight and bulk considerations need to be taken into account.
Prior art grating coupling geometry uses a corrugated metal/dielectric interface and when grating coupled in this wax the SPP propagates alone this corrugated boundary. Since the periodic surface may scatter energy associated with the mode into diffracted orders. the propagation length of the mode is reduced. The disadvantage is that complicated profiles cannot easily be made on a metal layer and expensive and complicated techniques of machining metal are required. In addition, the SPP that propagates along the textured surface may only be radiatively damped since the media either side of the boundary are usually non-absorbing.
It is an object of the invention to provide for a relatively thin, lightweight, broadband absorber, which is relatively simple to fabricate and incorporates a second damping mechanism by which the SPP may decay.
In a first aspect of the present invention, a low frequency, microwave or radar radiation absorber comprises a substrate having free charges and a dielectric layer coated onto said substrate surface, wherein said dielectric layer has a textured patterned surface so configured as to cause absorption of said incident microwave or radar radiation.
Preferably the substrate is metallic. Usually, the substrate is substantially planar and the textured surface is located on the upper surface of the dielectric layer.
Such dielectric gratings (wax) placed onto the metal plate will excite SPPs. The grating can potentially be far thinner than a quarter of a wavelength, and could even be applied in the form of sticky tapes at set spacing. Complicated profiles can easily be carved into soft dielectric (e.g. wax) layers.
In radiation absorbers according to the invention, there are two independent damping process that acts on the SPP as it propagates along the boundary. Firstly, the mechanism that allows radiation to couple into the SPP (i.e. the grating) will also allow the mode to radiatively decay. Secondly, although the top and bottom semi-infinite media (air and metal respectively) are effectively non-absorbing, at these frequencies, this may not be true for dielectrics, such as wax. Since the evanescent fields associated with the SPP mode penetrate the wax, any loss mechanisms within this overlayer will contribute a term to the damping of the mode. Both of these damping terms will contribute to the width of the surface plasmon resonance and will also have a similar effect on any guided modes propagating in the system.
Preferably the dielectric layer is doped with an appropriate absorbing material (e.g. ferrite particles, carbon fibre). In this instance, the SPPs are absorbed by the grating rather than the metal and absorption occurs across a range of wavelengths.
In a second aspect of the present invention, is a method of reducing the low frequency, microwave or radar radiation reflected/ retransmitted from an object comprising the steps of: arranging for the low frequency radiation to be incident on an article comprising a textured/patterned dielectric coated on a substrate having free charges; boosting the momentum of incident photons of the radiation to form surface plasmon polaritons at the substrate/dielectric interface; absorbing the energy of the incident photons by damping mechanisms.
The boosting of the momentum of incident photons occurs due to the textured/patterned surface of the dielectric. The damping mechanisms include a mechanism that allows radiation to couple into the SPP and loss mechanisms within the dielectric layer.